This invention relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) that exhibits improved impact and erosion resistance as a result of being a composite material containing a ceramic reinforcement material embedded in a ceramic matrix material.
Components within the hot gas path of a gas turbine engine are often protected by a thermal barrier coating (TBC) system. TBC systems include a thermal-insulating ceramic topcoat, referred to as the TBC, typically bonded to the component with an environmentally-protective bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides, notable examples of which are NiAl and NiAl(Pt). Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques.
TBC's employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation of the TBC. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten “splats,” resulting in a TBC characterized by flat (noncolumnar) grains and a degree of inhomogeneity and porosity that reduces heat transfer through the TBC.
While YSZ TBC's are widely employed for their desirable thermal and adhesion characteristics, they are susceptible to damage within the hot gas path of a gas turbine engine. For example, YSZ coatings are known to be susceptible to thinning from damage by particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The damage can be in the form of erosive wear (generally from smaller particles, lower particle velocities, and/or lower impingement angles) and impact spallation (generally from larger particles, greater particle velocities, and/or greater impingement angles). Impact damage and the resulting loss of TBC particularly occur along the leading edges of components such as turbine blades, while erosion is more prevalent on the concave and convex surfaces of the blades, depending on the particular blade design. Both forms of damage not only shorten component life, but also lead to reduced engine performance and fuel efficiency.
In commonly-assigned U.S. Pat. No. 5,683,825 to Bruce et al., an erosion-resistant TBC is disclosed in which alumina (Al2O3) or silicon carbide (SiC) is deposited as a protective coating on a TBC, or co-deposited with the TBC material to form a dispersion of particles in the TBC. Other examples of strengthening a TBC material through precipitate or particle dispersions include commonly-assigned U.S. Pat. No. 6,617,049 to Darolia et al. and U.S. Pat. No. 6,663,983 to Darolia et al., which disclose the inclusion of fine precipitates or particles on the order of up to five micrometers in diameter to provide a dispersion-hardening effect. Another use for fine precipitates in a TBC is taught in commonly-assigned U.S. Pat. No. 6,544,665 to Rigney et al., who disclose a TBC containing small amounts of alumina precipitates dispersed throughout its grain boundaries and pores to inhibit grain sintering and coarsening and pore coarsening that would lead to increased thermal conductivity.
Notwithstanding the above advances, there is still an ongoing need for TBC's that exhibit improved resistance to impact spallation and erosion for more demanding engine designs.